Device-to-Device (D2D) communication refers to direct communication between devices. In D2D communication, data to be transmitted from a first device to a second device is typically not relayed via any cellular network. Some examples of D2D communication of the prior art are Bluetooth communication, FlashlinQ communication, Wireless Local Area Network (WLAN, e.g. IEEE 802.11) communication (e.g. WIFI Direct).
Device-to-device communication may be applicable in various scenarios. One scenario is when a cellular radio access network is present, and able to set up a cellular connection between two devices. D2D communication may be a complement to the cellular communication in such scenarios.
There may be situations when D2D communication may provide better performance (better signal quality, higher bit rate, lower latency, etc) than cellular communication. This may be due to proximity between the devices and/or specific signaling gain of the D2D protocol (e.g. hop gain).
In some situations, the network may have constraints (e.g. due to being heavily loaded) resulting in that a service cannot be provided at all using a network connection. Then, D2D communication would be an alternative.
There may also be situations when D2D communication may be preferred by the user of a device (e.g. due to billing costs).
D2D communication may improve spectrum efficiency and reduce the network load for the cellular network, since the D2D connection typically uses another spectrum range (e.g. an unlicensed spectrum) than the cellular network (typically licensed spectrum). Furthermore, since cellular communication uses an uplink-downlink pair for each of the two devices while a D2D connection would only use one link pair, spectrum efficiency is improved even if the D2D connection would use cellular spectrum resources. This would be true even for network assisted D2D communication where most of the data would be transmitted over the D2D connection and only a small amount of information is to be transmitted over the network link.
D2D communication may be ad hoc or may be network assisted. For example, a cellular network may assist a D2D connection by establishing security of the D2D link and/or partly or fully controlling the setup of the D2D connection (e.g. device/peer discovery and resource allocation). A cellular network may also assist D2D communication by controlling the interference environment. For example, if using licensed operator's spectrum for the D2D communication, higher reliability can be provided than when operating in unlicensed spectrum. To assist the D2D connection, the network may also provide synchronization and/or partial or full Radio Resource Management (RRM—may e.g. comprise time and/or frequency resource allocation for the D2D communication).
In a typical cellular communication system, there are standardized procedures for connection setup and connection release (disconnection). There are typically also defined procedures for how to act in radio link failure (RLF), a scenario when a radio link is lost in an uncontrolled way (without connection release being performed). Such scenarios may be encountered e.g. when a device goes out of coverage, due to erroneous device operation, due to battery removal, etc.
Radio link monitoring procedures is typically defined for prior art cellular systems. Such procedures may include the device monitoring the radio channel and synchronization status with the network and define how to proceed of e.g. the synchronization is lost.
For example in Wideband Code Division Multiple Access (WCDMA) and Long Term Evolution (LTE), the wireless communication device monitors the down link (DL) channel conditions and determines whether the DL is in-sync or out-of-sync. This determination may be based on whether the device can reliably decode the DL information. If out-of-sync is detected, the device is to perform a number of actions, e.g. start higher layer connection release timers (indicating how long the device should try to re-synchronize before performing a formal connection release) and set transmission constraints (to limit the potential interference caused by the device if transmitting without reliable detection of DL power control information).